The present invention relates in general to an electrophotographic imaging apparatus and in particular to an electrophotographic apparatus capable of performing a printing operation wherein an electrophotographic imaging operation and a fusing operation are performed at different speeds.
In electrophotography, a latent image is created on an electrostatically charged photoconductive surface, e.g., a photoconductive drum, by exposing select portions of the photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner pattern is subsequently transferred from the photoconductive surface to the surface of a print medium, such as paper, which has been given an electrostatic charge opposite that of the toner.
A fuser then applies heat and pressure to the print medium before it is discharged from the apparatus. The applied heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
Fusing requirements may be more stringent when printing onto certain substrate types such as transparencies, compared to plain paper. For example, to produce good quality color transparencies, the un-fused opaque color toner components must be transparentized, which requires that all of the toner be adequately fused to the substrate. Also, more energy is required to fuse multiple layers of toner, e.g., for color printing, compared to fusing a single layer of toner, such as for monochrome printing because the fuser is required to fuse a much higher toner mass/area ratio. The fuser nip must also heat up the toner to a point that it flows on the surface of the transparency creating a smoothed substrate surface. The smoothed surface minimizes surface defects that can scatter light, making the image appear “dirty” or out of focus. Moreover, the smoothed surface allows light to transmit through the transparency and toner layer with very little diffusion. To address the above issues, fusing operations for transparencies generally require longer resident times of the substrate in the fuser compared to fusing operations for plain paper.
Color printers are typically optimized for printing at the highest operational speed. Unfortunately, the wide variation between the fastest print speed and the lower, optimal transparency print speed can cause motion quality artifacts in the electrophotographic operations formed at the lower speed, e.g., due to rotational velocity instability such as wow and flutter caused by operation of the electrophotographic motor at a non-optimized speed. In this regard, motors may be configured to tolerate relatively wide speed ranges using relatively complicated, multi-speed gearboxes to change the gear ratio when switching from high speed to low speed print jobs so that the motor operates within designed-for speed ranges. However, such a solution adds considerable cost, bulk and complexity to the system design.
Alternatively, a transfer device may be used as an intermediary to handoff the print medium, e.g., a transparency, from an image forming assembly to a fuser assembly. Under this configuration, the transfer device and the fuser assembly are both typically operated by a common fuser motor. Essentially, the image forming assembly is operated at a first, relatively high speed. The transfer device and the fuser assembly are ramped up to the first operating speed to accept a first handoff of the transparency from the image forming assembly to the transfer device. Once the transparency has cleared the transfer from the image forming assembly onto the transfer device, the operating speed of the transfer device and the fuser assembly are ramped down to a second, relatively slower speed that is optimal for fusing operations before a second handoff of the transparency from the transfer device to the fuser.
However, the above-described use of an intermediary increases the required inter-page gap between successive sheets thus reducing overall throughput of the electrophotographic device because the fuser motor speed, which also controls the transfer device, can not be ramped back up to the first speed until the trailing edge of the leading transparency has completely cleared the fuser nip. The result is that the overall print speed for transparencies is actually less than the optimized transparency fuser speed. For example, a printer may realize an output rate for transparencies of 6-7 pages per minute despite having the capability of operating at a fusing rate of approximately 10 pages per minute because the inter-page gap between successive transparencies must be increased to accommodate the time required for ramping up the transfer device for the first handoff and subsequently slowing down the transfer device for the second handoff.
Further, the image forming assembly of a conventional printer typically comprises a toner cartridge having a developer roll that turns against a corresponding photoconductive drum to supply the drum with toner. Toner is stripped off the developer roll and is recycled back to the cartridge if such toner is not transferred to the drum surface as the drum and developer roll rotate. However, repeated recycling or churning of the toner begins to strip electrophotographic additives from the toner, thus decreasing the useful life of the toner particles. The drum and the developer roll typically rotate during an entire printing operation, including the time required to ramp up and ramp down the transfer device, e.g., when printing transparencies as noted above. During such ramp up and ramp down times, the drum is not printing, e.g., directly onto a print medium or an intermediate transfer member belt, and is not removing toner from the developer roll, thus increasing the amount of toner churn.